Time of flight mass spectrometer having a flowing gas stream perpendicular to the ion drift field for increased resolution

ABSTRACT

Apparatus and methods for sorting and detecting trace gases which undergo ion-molecule reactions. Positive or negative ions of the trace gas are formed by ion-molecule reactions between the molecules of the trace gas and primary ions from another gas. Ions are classified in accordance with their velocity in a stream of gas while subjected to an electric drift field.

United States Patent [72] Inventors Martin J. Cohen [50] Field of Search 250/4 l .J 1, West Palm Beach; 41.96, 44, 83.6 FT, 41.); 55/2. 3. 17. It)! David 1. Carroll, Lantana; Roger F. Wernlund, Lake Worth, all of Fla. References Cited [21] Appl. No. 779,097 UNITED STATES PATENTS 1 Filed Nov-26,1968 2,950,387 8/1960 Brubaker 250/4191 1 Patented Nov-30, 1971 3,254.209 5/1966 Tite etal 250/4191 SB [73] Assignee Franklin Gno Corporation v West Palm Beach, Fla Pnmary Exammer-James W. Lawrence Asxisiant ExaminerC. E. Church All0rny Raphael Semmes [54] TIME OF FLIGHT MASS SPECTROMETER FIELD FOR ABSTRACT: Apparatus and methods for sorting and detect- INCREASED RESOLU-HON ing trace gases which undergo ion-molecule reactions. Posi- 29 Claims, 5 Drawing Figs live or negative ions of the trace gas are formed by ionmolecuie reactions between the molecules ofthe trace gas and [52] U.S. Cl 250/419 primary i f another gas. Ions are classified in [51 Int. Cl BOld 59/44, Cordance with their velocity in a Stream of gas while Subjected U 39/34 to an electric drift field.

RECYCLE SYSTEM PATENTEnuuvsolsn 3.624.389

FIG. 4

INVENTURS MARTIN J. COHEN DAVID L CARROLL ROGER F. WERNLUND ATTORNEY PATENTED NUVBOISYI SHEET 2 UP 2 FIG. 3

RECYCLE SYSTEM INVENTORS MARTIN J. COHEN DAVID 1. CARROLL ROGER F. WERNLUND QMM ATTORNEY TIME OF FLIGHT MASS SPECTROMETER HAVING A FLOWING GAS STREAM PERPENDICULAR TO THE ION DRIFT FIELD FOR INCREASED RESOLUTION BACKGROUND OF THE INVENTION This invention relates to apparatus and methods of ion classification utilizing gas flow as a measurement parameter. More particularly, the invention is concerned with the detection of trace vapors which undergo ion molecule reactions and with the separating and measuring of molecular quantities of trace substances in a gaseous sample.

It has heretofore been proposed to measure ion mobility by ionizing molecules of a stream of gas and subjecting the ions to an electric drift field, which may be transverse to the gas flow direction or opposed to the gas flow. Reference is made, for example, to Physical Review, Feb., 1929, Vol. 33, p. 217 et seq. More recently the standard electron capture detectors have utilized airflow in a rough way to separate electrons and ion components of current for purposes of detection of the reduction in current when electron carriers attach to become ions. However, simple and practical instruments employing gas flow as a measurement parameter have not heretofore been available for high-sensitivity, high-resolution ion detection and classification.

BRIEF DESCRIPTION OF THE INVENTION It is accordingly a principal object of the present invention to provide improved apparatus and methods. employing gas flow as a quantitative parameter in the detection of trace gases which are capable of being electrically charged.

A further object of the invention is to provide improved apparatus and methods for separating and measuring molecular quantities of trace substances.

Briefly stated, preferred embodiments of the apparatus and methods of the invention are concerned with Plasma Chromatography" systems involving the formation of either positive or negative ions by reactions between the molecules of the trace substances and primary or reactant ions. The secondary or product ions may then be separated, detected, and measured. Separation is accomplished by utilizing the difierence in velocity of ions of different mass in an electric field applied to a stream of gas into which the ions are injected. In a preferred form of the invention primary ions are formed by electron attachment, for example, to the molecules of a reactant gas. A drift field causes the primary ions to migrate through a reaction chamber, during which the primary ions react with molecules of a trace gas to be detected to form secondary trace gas ions, The secondary ions and any remaining primary ions pass through an ion-transmissive aperture into a stream of gas and drift transversely of the stream. Ions having a predetermined mobility follow a path which leads to an ion detector, while ions of different mobility follow different paths and do not reach the detector.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be further described in conjunction with the accompanying drawings, which illustrate preferred and exemplary embodiments, and wherein:

FIG. I is a plan view of apparatus in accordance with the invention;

FIG. 2 is a somewhat diagrammatic side elevation view, partly in section, illustrating the apparatus of the invention;

FIG. 3 is a transverse sectional view taken along line 3-3 of FIG. 2;

FIG. 4 is a contracted diagrammatic plan view illustrating a modification of the invention; and

FIG. 5 is a vertical sectional view taken along line 5-5 of FIG. 4.

DETAILED DESCRIPTION OF THE INVENTION The copending application of Martin J. Cohen, David I, Carroll, Roger F. Wernlund, and Wallace D. Kilpatrick, filed Oct. 23, 1968 and entitled Apparatus and Methods for Separating, Concentrating, Detecting and Measuring Trace Gases," discloses Plasma Chromatography systems involving the formation of primary ions and the reaction of such primary ions with molecules of trace substances to form secondary ions, which may be concentrated, separated, detected, and measured by virtue of the difference in velocity or mobility of the ions in an electric field. As set forth in the said copending application, the broad principle of ion-molecule reactions is well documented in the literature, but the utilization of this principle in high-sensitivity systems for detecting and measuring trace substances is novel. Primary ions may be produced by subjecting the molecules of a suitable host gas, such as air, to ionizing radiation, such as beta rays from a tritium source, corona from a multipoint or wire array, electrons produced by photoemission from a cathode, etc. The primary ions are subjected to an electric drift field, causing them to migrate in a predetermined direction through a reaction space into which the sample or trace gas is introduced. The resultant collisions between the primary ions and the molecules of the sample gas produce secondary ions of the sample gas in much greater numbers than can be produced by mere electron attachment, for example, to the trace gas molecules. The secondary ions are also subjected to the electric drift field and may be sorted in accordance with their velocity or mobility. The specific systems of the said copending application employ ion shutter grids or gates for segregating the ion species in accordance with their drift time. In the present invention the secondary ions are injected into a stream of gas and are sorted in accordance with their velocity under the combined influence of the drift field and the gaseous flow.

Referring to the drawings, apparatus in accordance with the preferred form of the invention comprises a duct or tunnel 10 of rectangular cross section, having a first pair of parallel sides 12 and 14, which may be termed the top and bottom, and a second pair of parallel sides 16 and 18, which may merely be termed sides. The duct has an inlet section 20 and an outlet section 22. The inlet section has a transition portion 24 which contracts transversely in two dimensions from the upstream to the downstream end of the transition portion. The main section of the duct between the inlet and outlet sections is of uniform rectangular cross section. The outlet section 22 may contract transversely in a horizontal plane, as indicated by the transition portion 26 in FIG. I, and may expand in a vertical plane as shown in FIG. 2. The outlet section may include an isolation sleeve 28 leading to a housing 30 containing a blower, which may exhaust to the atmosphere or which may be part ofa recycling system indicated diagrammatically at 32 in FIG. 2.

The top and bottom walls 12 and 14 of the main section of the duct are preferably of conductive material, while the remaining sidewalls l6 and 18 are preferably constituted by alternating strips of conductive material 34 and insulation 36. From FIG. I it can be observed that the structure comprising the alternate strips 34 and 36 is external to the main gas flow passage through the duct and hence does not produce turbulence in the duct, the sidewalls of which are essentially smooth and continuous. The inlet section 20 contains a series of parallel screens or grids 38 arranged transversely of the gas flow and serving to reduce turbulence. The use of screens 38 together with the transition portion 24 (which preferably has a cross section contraction ratio of about 4), the smooth sidewalls of a relatively long main duct section, and the use of relatively low gas flow velocity reduce turbulence and eddies to a minimum in order to preserve uniform gas flow velocity profile through the main section of the duct.

Adjacent to the top of the duct and external of the main gas flow passage is an ion-molecule reaction chamber 40 into which a sample and host gas may be introduced by means of an inlet conduit 42. Chamber 40 communicates with the interior of the duct through a small slit or ion-transmissive window 44 formed in a conductive portion of the top wall 12 between a pair of spaced insulator portions 48. Chamber 40 contains a principal electrode 50 (a cathode if negative ions are produced or an anode if positive ions are produced) which may have an opening 52 through which gas may enter the chamber space. A source of ionizing electric charge, such as a tritium strip on electrode 50, is provided in the region of the electrode. Arcuate focusing grids are provided at 54 and 56. Gas may exhaust from the chamber 40 through a separate duct 58 running external of the top wall 12 into the outlet 22 or exiting to the atmosphere. The relative pressures in the chamber 40 and the duct 10 may be maintained by standard flow control techniques so as to minimize the leakage of neutral molecules through slit 44. If desired, the slit may be covered by a thin ion-permeable membrane.

The bottom wall 14 is provided with a slit 60 which leads to a dead air space 62 in which is located an ion sensor, which may comprise electrode 64 of an electrometer-type ion detector. The main section of the duct 10 may be divided longitudinally by a central plate or diaphragm 66 parallel to the top and bottom walls 12 and 14 and having a transverse opening 68 for the passage of ions. The diaphragm assists in preserving uniformity of the electric drift field which will now be considered.

The electric drift field may be produced and maintained uniform by applying appropriate electric potentials to electrode 50, top wall 12, bottom wall 14, electrometer electrode 64, and the conductive strips 34, which constitute electric guard slats. Any suitable DC supply provided with a voltage divider, such as a resistor chain, may be employed. When negative ions are produced, the bottom plate 14 of the duct may be at ground potential, the top plate 9,000 volts negative relative to ground, the cathode 50 9,100 volts negative relative to ground, the electrometer electrode 64 lOO volts positive relative to ground, and the guard slats 34 at voltages increasingly negative with respect to ground (within the O to -9,000 volt range) from the bottom wall 14 to the top wall 12. For use with positive ions the polarity is reversed. if the top and bottom walls proper are electrified, rather than separate electrode sections, the ends of these walls must obviously be insulated from each other.

In the operation of the apparatus of the invention, a host gas carrying an ionizable gaseous trace sample, for example air carrying S or the insecticide Ethion, enters the reaction chamber 40 through the inlet 42. The host gas, or components thereof such as oxygen or nitrogen, is ionized adjacent to electrode 50 by the source of ionizing electric charge. As pointed out in the said copending application, ions of the plentiful host gas molecules form preferentially. The primary ions then migrate toward the aperture 44 under the influence of the drift field, the voltage gradient being relatively low in the reaction chamber. In traversing the reaction chamber 40 the primary ions undergo ion-molecule reactions with the molecules of the trace gas, producing secondary ions of the trace gas. The resultant ions are focused upon aperture 44 by grids 54 and 56 and pass through the aperture into the stream of gas drawn through duct by the blower. The ions drift downstream and toward the bottom wall 14 under the combined influences of the gas flow and the drift field, the path followed by the ions being a function of their velocity. Some of the ions will follow paths ending upon the bottom wall 14, where the ions will be neutralized. Predetermined species of ions, depending upon ion mass, gas flow velocity, and drift field parameters, will pass through aperture 60 and impinge upon electrode 64, producing an output current which is measured by a suitable detection circuit, such as an integrator. By virtue of the utilization of the ion-molecule reaction principle to produce large numbers of trace ions and by utilizing low-velocity uniform gas flow profile, high sensitivity and high resolution are attainable. Detcctable output currents for particular ion species are produced at significantly lower trace concentration than with comparable apparatus known heretofore.

The gas employed in the duct or tunnel 10 may be cleaned, adjusted in temperature and pressure, and flow controlled by the recycle system 32. Furthermore, the tunnel gas may be different from the host gas supplied to the reaction chamber and may be inert with respect to ionmolecule reactions involving the primary and secondary ions of interest, thereby to serve as a quenching medium for terminating ion-molecule reactions and for restricting the same to the chamber 40. The tunnel gas may be recycled separately from the host and sample gases, which may have their own pump or pressure source. The use of an ion-reaction region within chamber 40 which is quite small permits rapid sample change from a sample source with a small total quantity of available trace, such as a gas chromatograph.

FIGS. 4 and 5 illustrate a modification of the invention especially adapted to gas chromatograph use. The apparatus is essentially the same as previously described except that the reaction chamber 40' is provided with separate sources of sample and reactant gases. An inert gas carrier containing a trace of sample may be introduced to the reaction chamber 40 through conduits 42' of an inlet manifold, while a reaction gas may be separately introduced through conduits 42" of a separate inlet manifold. The outlet manifold is indicated generally at 58' and may include separate conduits leading to the blower. If the conduits from the inlet to the outlet manifolds are continuous, conduit perforations may be employed for admitting gas to the reaction chamber space. In the embodiment of FIGS. 4 and 5, the gas introduced through inlet conduits 42 may be the inert carrier gas and sample output from a gas chromatograph, while the reaction gas may be any suitable primary ion-forming gas.

As indicated above, in accordance with the invention ion species are separated under the combined influence of an electric drift field and a gaseous stream. The ion velocity in the field direction is equal to the ion mobility multiplied by the field strength. The ion velocity in the gas flow direction is equal to the flow velocity. The resultant ion velocity is the vector sum of these two components. The velocity of the ion in the gas flow at atmospheric pressure is independent of the ion mass. The velocity of the ion in the electric field is dependent upon the mobility, which is a function of both the ion and carrier gas masses. The mobility at very low concentrations in air is given for normal gases by the Langevin relationship:

K =B lH-m lm where B is an empirical constant, m, the mass of the carrier gas molecules, and m the mass of ions. For air m, is 28.55 and B is approximately 1.96. The ion velocity vector due to the electric field is thus:

V =l.96E l+28.55/m, where E is the electric field. Sinde the electric field velocity vector is dependent upon the ion mass, the resulting vector sum will be also. The transit time of an ion between the top and bottom wall electrodes of the duct is independent of gas flow velocity and is equal to the electrode spacing divided by V The transit time multiplied by the gas flow velocity V will give the distance travelled by the ion perpendicular to the electric field. Thus if the ion receiver slot is positioned downstream of the ion injection slot by a distance appropriate to the desired ion mass, it will receive only ions of the selected mass. Ions of lower mass will not travel far enough to reach the receiver slot, while ions of higher mass will travel too far to reach the slot.

In a typical apparatus of the type illustrated in the drawings the inlet section may be 23 inches wide by 13 inches high, 7 inches long for the uniform cross dimension portion, and 8 inches long for the transition section. The length of the main duct section may be 40 inches long, the internal width 16 inches and the height 5 inches. Slit 44 may be located at about the longitudinal center of the top wall of the main duct section. The center of slit 68 may be displaced downstream 2.5 inches from the center of slit 44, while the center of slit 60 may be displaced downstream 2.5 inches from the center of slit 68. The width of slit 44 may be 0.040 inch. The thickness of the sides of the duct constituted by the alternate strips of conductive and nonconductive material may be 2 inches. Typically the ion source may be a 7 square centimeter strip of tritium, the airstream velocity l,000 centimeters per second, the

electric drift field between the top and bottom walls of the duct 760 volts per centimeter, the working pressure 760 torr. Sensitivity to a selected trace material may be 1 part in The ion velocity filter approach of the present invention offers the potential for much higher sensitivities, by virtue of the continuous ion source, than is possible with pulsed ion sources. Another advantage is that all ions which do not enter the collector slot are intercepted by the adjacent anode (or cathode) plate and eliminated from the signal.

Among the many applications of the invention is the analysis of stack gas for S0 and 80;, content. In stack gases most sulfur already exists in the form of S0 and S0 The remainder, if any, can be converted to S0 by a simple catalytic oxidation flow cell, A small volume of the stack gas is obtained from across the stack by a rack-type inhaler to obtain a representative sample. The sample is filtered of gross particulate and passed through a heated catalytic oxidizer to convert any remaining sulfur aerosol, hydrogen sulfide and carbon-sulfur compounds to sulfur dioxide. The gases may they be diluted 10 times or more with fresh air and inserted into the reaction chamber.

While preferred embodiments of the invention have been shown and described, it will be apparent to those skilled in the art that changes can be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims.

The invention claimed is:

l. A method of detecting a substance in a gaseous sample, which comprises forming reactant ions, reacting said reactant ions with molecules of said sample to form product ions of different mobility including ions of said substance, inserting said product ions at a first region in a substantially nonturbulent continuous gaseous fiow of predetermined flow rate while applying an electric drift field to said product ions transversely of the flow to cause the product ions to follow paths dependent upon their mobility, selectively sensing the product ions of said substance which follow a predetermined path to a second region spaced from the first region transversely of said flow in the direction of said filed and downstream of the first region, and producing an electrical output in response to the second ions.

2. A method in accordance with claim 1 and in which unreacted reactant ions, as well as the product ions, are inserted in the gaseous flow with said field applied thereto to cause the ions to follow paths dependent upon their mobility.

3v A method in accordance with claim 1, and in which the formation of theproduct ions occurs in a first gaseous medium and the insertion of the product ions in the gaseous flow occurs in a second gaseous medium.

4. A method in accordance with claim 3, and in which the second gaseous medium is recycled.

5. A method in accordance with claim 3, and in which the second gaseous medium is inert with respect to ion-molecule reactions involving the reactant product.

6. A method in accordance with claim 1, and in which the ion-molecule reaction occurs in a chamber and the gaseous flow occurs in asubstantially larger chamber.

7. A method in accordance with claim I, and in which the electric drift field is substantially perpendicular to the direction of gaseous flow.

8. A method in accordance with claim 2, and in which the formation of the product ions occurs in a first gaseous medium and the insertion of the product ions and unreacted reactant ions in the gaseous flow occurs in a second gaseous medium.

9. A method in accordance with claim 1, and in which the sample is carried by an inert carrier gas and the reactant ions are formed from a separate reactant gas.

10. Apparatus for detecting a trace substance in a gaseous sample, which comprises a duct having means for producing a substantially nonturbulent continuous gas flow therethrough at a predetermined rate, a chamber adjacent to said duct, said chamber having means for receiving a gaseous sample containing said trace substance and having means associated therewith for producing reactant ions which react with the molecules of said trace substance to form product ions, at least some of said ions having different mobility, means for introducing said product ions into said duct at a first region, means for applying an electric drift field to the product ions introduced into said duct from said chamber, said field being transverse to said flow, and means adjacent to another region of said duct spaced transversely from the first region in the direction of said field and located downstream of the first region for selectively sensing ions which reach said other region by following paths dependent upon their mobility in the gaseous flow in said duct and the drift field.

11. Apparatus in accordance with claim 10 wherein said reactant-ion-producing means comprises a continuous ionizing source.

12. Apparatus in accordance with claim 10, and in which the firstmentioned means comprises an inlet to said duct with a portion which contracts transversely in a downstream direction.

13. Apparatus in accordance with claim 12, and in which the ratio of contraction of said inlet portion from its upstream to its downstream end is substantially four.

14. Apparatus in accordance with claim 12, and in which said duct is of rectangular cross section and said inlet portion contracts both in height and width.

15. Apparatus in accordance with claim 12, and in which said inlet has a series of transversely disposed turbulencereducing screens located upstream of its contraction portion.

16. Apparatus in accordance with claim 10, and in which said chamber is located exteriorly of said duct at one side thereof and said sensing means is located at the opposite side thereof.

17. Apparatus in accordance with claim 16, in which said duct is of rectangular cross section and in which each of the remaining sides of said duct comprises alternating strips of electrically conductive and electrically insulating material parallel to the first-mentioned sides and located outside of the flow path through said duct.

18. Apparatus in accordance with claim 17, and in which said duct is provided with a longitudinally extending central plate parallel to the first-mentioned sides of the duct and having an ion-transmissive aperture therein for passage of ions from said chamber to said sensing means.

19. Apparatus in accordance with claim 10, and in which said sensing means comprises a dead air chamber external to said duct and coupled to the interior of said duct by an iontransmissive aperture.

20. Apparatus in accordance with claim 10, and in which said means for applying said drift field comprises a pair of electrodes at opposite sides of said duct at said regions, respectively.

21. Apparatus in accordance with claim 10, and in which said chamber has means for applying a drift field to ions therein to urge them toward said duct.

22. Apparatus in accordance with claim 10, further comprising means for recycling gas through said duct.

23. Apparatus in accordance with claim 10, further com prising means for introducing a sample gas and a reaction gas into said chamber separately.

24. Apparatus for ion detection, which comprises an elongated duct having a pair of parallel electrodes at opposite sides thereof, having means for providing a substantially nonturbulent, continuous stream of gas therethrough at a predetermined rate, and said means comprising an inlet section which contracts transversely in a downstream direction, means for introducing ions into said duct at a region adjacent to one electrode, and means for selectively sensing ions at a region adjacent to the other electrode downstream of the first-mentioned region.

25. Apparatus in accordance with claim 24, and in which said inlet section contains transversely disposed turbulencereducing grid means.

26. Apparatus in accordance with claim 24, and in which said duct is of rectangular cross section and the remaining sides comprise longitudinally extending alternate strips of electrically conductive and electrically nonconductive material.

27; Apparatus in accordance with claim 10, wherein said ion-introducing means comprises means for focusing product ions upon said first region.

28. A method of sorting ions which comprises forming different mobility ions of a sample substance at a first region by ion-molecule reaction between the molecules of the substance and reactant ions, inserting the sample ions into a substantially nonturbulent continuous gas stream of predetermined flow rate at a second region, subjecting said sample ions to an electric drift field transverse to said gas stream while they are in said stream, whereby the sample ions drift to locations dependent upon their mobility, selectively sensing the sample ions which reach a predetermined location transversely of said stream in the direction of said field and downstream of said second region, and producing am electrical output in response to the sensed ions.

29. A method in accordance with claim 28, wherein the subjecting of said sample ions to said drift field comprises applying to said sample ions a steady unidirectional drift field during the other steps recited.

* m nt e 

1. A method of detecting a substance in a gaseous sample, which comprises forming reactant ions, reacting said reactant ions with molecules of said sample to form product ions of different mobility including ions of said substance, inserting said product ions at a first region in a substantially nonturbulent continuous gaseoUs flow of predetermined flow rate while applying an electric drift field to said product ions transversely of the flow to cause the product ions to follow paths dependent upon their mobility, selectively sensing the product ions of said substance which follow a predetermined path to a second region spaced from the first region transversely of said flow in the direction of said field and downstream of the first region, and producing an electrical output in response to the sensed ions.
 2. A method in accordance with claim 1 and in which unreacted reactant ions, as well as the product ions, are inserted in the gaseous flow with said field applied thereto to cause the ions to follow paths dependent upon their mobility.
 3. A method in accordance with claim 1, and in which the formation of the product ions occurs in a first gaseous medium and the insertion of the product ions in the gaseous flow occurs in a second gaseous medium.
 4. A method in accordance with claim 3, and in which the second gaseous medium is recycled.
 5. A method in accordance with claim 3, and in which the second gaseous medium is inert with respect to ion-molecule reactions involving the reactant and product ions.
 6. A method in accordance with claim 1, and in which the ion-molecule reaction occurs in a chamber and the gaseous flow occurs in a substantially larger chamber.
 7. A method in accordance with claim 1, and in which the electric drift field is substantially perpendicular to the direction of gaseous flow.
 8. A method in accordance with claim 2, and in which the formation of the product ions occurs in a first gaseous medium and the insertion of the product ions and unreacted reactant ions in the gaseous flow occurs in a second gaseous medium.
 9. A method in accordance with claim 1, and in which the sample is carried by an inert carrier gas and the reactant ions are formed from a separate reactant gas.
 10. Apparatus for detecting a trace substance in a gaseous sample, which comprises a duct having means for producing a substantially nonturbulent continuous gas flow therethrough at a predetermined rate, a chamber adjacent to said duct, said chamber having means for receiving a gaseous sample containing said trace substance and having means associated therewith for producing reactant ions which react with the molecules of said trace substance to form product ions, at least some of said ions having different mobility, means for introducing said product ions into said duct at a first region, means for applying an electric drift field to the product ions introduced into said duct from said chamber, said field being transverse to said flow, and means adjacent to another region of said duct spaced transversely from the first region in the direction of said field and located downstream of the first region for selectively sensing ions which reach said other region by following paths dependent upon their mobility in the gaseous flow in said duct and the drift field.
 11. Apparatus in accordance with claim 10 wherein said reactant-ion-producing means comprises a continuous ionizing source.
 12. Apparatus in accordance with claim 10, and in which the first-mentioned means comprises an inlet to said duct with a portion which contracts transversely in a downstream direction.
 13. Apparatus in accordance with claim 12, and in which the ratio of contraction of said inlet portion from its upstream to its downstream end is substantially four.
 14. Apparatus in accordance with claim 12, and in which said duct is of rectangular cross section and said inlet portion contracts both in height and width.
 15. Apparatus in accordance with claim 12, and in which said inlet has a series of transversely disposed turbulence-reducing screens located upstream of its contraction portion.
 16. Apparatus in accordance with claim 10, and in which said chamber is located exteriorly of said duct at one side thereof and said sensing means is located at the opposite side thereof.
 17. Apparatus in accordance With claim 16, in which said duct is of rectangular cross section and in which each of the remaining sides of said duct comprises alternating strips of electrically conductive and electrically insulating material parallel to the first-mentioned sides and located outside of the flow path through said duct.
 18. Apparatus in accordance with claim 17, and in which said duct is provided with a longitudinally extending central plate parallel to the first-mentioned sides of the duct and having an ion-transmissive aperture therein for passage of ions from said chamber to said sensing means.
 19. Apparatus in accordance with claim 10, and in which said sensing means comprises a dead air chamber external to said duct and coupled to the interior of said duct by an ion-transmissive aperture.
 20. Apparatus in accordance with claim 10, and in which said means for applying said drift field comprises a pair of electrodes at opposite sides of said duct at said regions, respectively.
 21. Apparatus in accordance with claim 10, and in which said chamber has means for applying a drift field to ions therein to urge them toward said duct.
 22. Apparatus in accordance with claim 10, further comprising means for recycling gas through said duct.
 23. Apparatus in accordance with claim 10, further comprising means for introducing a sample gas and a reaction gas into said chamber separately.
 24. Apparatus for ion detection, which comprises an elongated duct having a pair of parallel electrodes at opposite sides thereof, having means for providing a substantially nonturbulent, continuous stream of gas therethrough at a predetermined rate, and said means comprising an inlet section which contracts transversely in a downstream direction, means for introducing ions into said duct at a region adjacent to one electrode, and means for selectively sensing ions at a region adjacent to the other electrode downstream of the first-mentioned region.
 25. Apparatus in accordance with claim 24, and in which said inlet section contains transversely disposed turbulence-reducing grid means.
 26. Apparatus in accordance with claim 24, and in which said duct is of rectangular cross section and the remaining sides comprise longitudinally extending alternate strips of electrically conductive and electrically nonconductive material.
 27. Apparatus in accordance with claim 10, wherein said ion-introducing means comprises means for focusing product ions upon said first region.
 28. A method of sorting ions which comprises forming different mobility ions of a sample substance at a first region by ion-molecule reaction between the molecules of the substance and reactant ions, inserting the sample ions into a substantially nonturbulent continuous gas stream of predetermined flow rate at a second region, subjecting said sample ions to an electric drift field transverse to said gas stream while they are in said stream, whereby the sample ions drift to locations dependent upon their mobility, selectively sensing the sample ions which reach a predetermined location transversely of said stream in the direction of said field and downstream of said second region, and producing am electrical output in response to the sensed ions.
 29. A method in accordance with claim 28, wherein the subjecting of said sample ions to said drift field comprises applying to said sample ions a steady unidirectional drift field during the other steps recited. 